It is known to provide apparatus for measuring the surface or the profile of an object, to derive the texture or roughness or shape thereof. One example of such apparatus is the FORM TALYSURF.TM. measuring system available from Rank Taylor Hobson Limited, P O Box 36, 2 New Star Road, Leicester LE4 7JQ, UK. That apparatus comprises a probe member, or stylus, projecting downwardly to contact the surface to be measured and provided adjacent one end of a supporting arm mounted pivotally to a supporting structure.
The supporting structure is mounted for linear motion parallel (or approximately so) to the surface of the object to be measured, and is moved across the surface in a line by a drive system. The supporting arm extends beyond the pivotal mounting and carries a reflective surface defining one end of one of two optical paths along which a collimated light source (a laser) is directed.
At a reference probe position above the surface, the two path lengths are equal. When the surface height varies, however, the probe is urged by gravity to follow the surface and the reflective surface at the other end of the pivoted support arm travels, changing the optical path length of one of the paths and thus generating a fringe pattern of interference.
Counting the number of fringes which pass a given optical detector position provides a measure of the displacement of the probe. Fringe counting apparatus is therefore provided to continually count the fringes passing a fixed detector position and to provide therefrom an output signal representing the probe displacement above the surface to be measured whilst the probe structure is moved linearly across the surface. It would, of course, be equally possible to move the surface, rather than the probe.
To provide a measurement of the roundness or profile of a rotatable object such as a crank shaft or an axle, the object is positioned beneath the stylus and mounted to be rotatably driven. As the object rotates, the probe measures the circumference of the object, from which any eccentricity or deviation from a desired profile can be detected.
Such measuring apparatus is required to have extremely high accuracy in measuring the probe position (and hence the height of the surface). The above described apparatus can achieve a displacement resolution on the order of 10 nm. Another important property of such apparatus is the maximum displacement which the stylus can measure; this needs to be reasonably large for measurement of many types of surface or profile, and typically on the order of millimeters. A useful measure of the performance of such measurement equipment is the "dynamic range", defined as range R(mm)/resolution R(mm). This should preferably be as high as possible.
Whilst the measuring apparatus above provides excellent performance, a number of problems can arise. Firstly, the Michelson type interferometer used measures the difference in optical path length between the two optical paths provided. It depends critically upon a stable light wavelength, whereas changes in atmospheric pressure and temperature can cause changes in the wavelength of light and hence lead to incorrect measurements. Since the two optical paths can have widely differing lengths, the light source must have a very long coherence length; providing a suitable light source thus requires an expensive and bulky type of laser, requiring a high voltage supply and involving considerable dissipation of heat.
U.S. Pat. No. 3,726,595, FIG. 8-1, shows a surface measurement apparatus which employs instead a grating interferometer. In a grating interferometer, a light beam illuminates a grating and is diffracted thereby to produce a pair of first order diffracted beams (although higher orders could be used). The two beams are reflected so as to travel equal path lengths and are recombined to provide an interference pattern. When the grating moves laterally, the path of each beam remains constant but the phase of each beam is changed, so that the fringes of the interference pattern are shifted. The motion of the fringes therefore provides a measure of the lateral motion of the grating.
In U.S. Pat. No. 3,726,595, the grating is positioned normal to a surface and carries a probe at its surface engaging end, so that when the interferometer is moved laterally along the surface the grating is forced to move normal to the surface and the resulting variation in the interference pattern gives a measurement of the probe position.
Inherent in the nature of this apparatus is the necessity that the grating should be constrained to move purely linearly, transverse to the illuminating beam and to the line bisecting the two diffracted order beams. However, in the measurement of rough or irregular surfaces or, in general, surfaces which include rising edges encountered by the probe, this method of mounting the probe would be unsatisfactory since when the probe were pulled into contact with such a rising edge, compressive stress within the probe would be set up as the probe moved over the edge and this would, firstly, tend to shift the alignment of the grating (disrupting the interference pattern), and secondly tend to cause the probe to vibrate due to the increased friction with the surface. It would also tend to put increased stress on the probe mounting.